The present invention relates to an aligning method which ensures a high precision of alignment and which is suited to the aligning apparatus for producing .Iadd.a .Iaddend.large-scale integrated semiconductor device (referred to as "LSI", hereinunder). The invention is concerned also with an exposure method for making a transfer of a pattern by the aligning method mentioned above.
FIGS. 1a and 1b show an example of conventional aligning methods. More specifically, FIG. 1a shows an example of an aligning pattern on a photo mask. In this example, a transfer pattern .alpha.1 for the aligning pattern is formed on a wafer by means of radial lines of a constant line width, while FIG. 1b shows how a further alignment is made on the aligning pattern formed on the wafer with the same pattern. Namely, the shadow .alpha.2 of an aligning mark formed on a photo mask is aligned with the pattern .alpha.1 formed already on the wafer. This aligning method can provide a degree of aligning precision on the order of .+-.0.3 .mu.m which is quite unacceptable for the alignment of LSI having a gate length of less than 1 micron. In fact, LSIs of 0.5 micron rule require a high degree of aligning precision of 0.05 .mu.m which can never be achieved by the conventional aligning method.
Austin et al proposes, in Applied Physics Letters Vol. 31, No. 7, p. 428, 1977, an aligning method making use of a double grid. In this method, as shown in FIG. 2, an incoming laser beam is applied to a photo mask 2 and is diffracted by a grid 3 formed on the photo mask 2. The diffracted ray is diffracted once again by a grid 5 formed on the wafer 4 to obtain diffracted ray beans 6, 7, 8 . . . These diffracted ray beams 6, 7 and 8 can be expressed as (0, 1), (1, 1), (-1,2) . . . by binary representation in terms of the order of diffraction on the photo mask and the order of diffraction on the wafer. These diffracted beams are focussed on a point by means of a lens for the measurement of the ray intensity. The diffracted ray beam has a ray intensity distribution in bilateral symmetry with respect to the incoming laser beam 1, so that the alignment of the photomask 2 with the wafer 4 can be attained by making the intensities of diffracted beams observed on both sides equal together. It is said that this aligning method can provide a degree of aligning precision of several hundreds of .ANG.. In this method, however, the alignment between the photo mask 2 and the wafer 4 is largely affected by the distance D between the photo mask 2 and the wafer 4, so that it is necessary to control the distance D with a high degree of accuracy. In addition, this method requires an impractically complicated apparatus because the aligning operation has to be made with the photo mask 2 and the wafer 4 being positioned close to each other with precise control of the distance D therebetween.
In order to align elements having a line width of sub-micron order, it has been proposed to observe the discharge of secondary electron from the elements This, method, however, is also impractical because this method cannot be carried out in the atmosphere and, hence, the through-put is too small in the production of LSI.